Field of the Invention
The invention relates to the use of organopolysiloxane copolymers as defoamers for aqueous coating systems and printing inks.
Increasing numbers of coating materials and printing inks are being reformulated on an aqueous basis for greater environmental acceptability.
Because of the ingredients used—emulsifiers, wetting agents, and dispersing additives—such formulations have a strong tendency to stabilize foam. This is manifested to adverse effect not only during the production of these paints and inks but also more particularly during their application, when large quantities of air are introduced and the esthetics and physical properties of the final coatings are impaired. Consequently, in virtually all water-based systems the use of antifoams or defoamers is widespread and often indispensable.
In the past a large number of formulations have been described which envisage the use of, for example, silicone oils, organically modified siloxanes, hydrophobic polyoxyalkylenes, mineral oils, natural oils, and other hydrophobic liquids as defoaming substances. Frequently combinations of the abovementioned substances with hydrophobic solids, such as silicas, metal stearates or fatty acid amides, for example, are also used, which often reinforce the foam-inhibiting or defoaming effect.
The use of silicone oils, especially dimethylpolysiloxanes of low to medium viscosity, for the defoaming of aqueous solutions or dispersions is known and is described in, for example, the book by W. Noll “Chemie and Technologie der Silicone”.
It is likewise known to use polyoxyalkylene-polysiloxane copolymers as defoamers. U.S. Pat. No. 3,763,021 describes a typical preparation for defoaming of aqueous latices, consisting of    (1) from 1 to 20% by weight of a siloxane glycol copolymer of the general formula      in which            x has an average value of from 6 to 420 and        y has an average value of from 3 to 30, a        G is a radical of the structure—D(OR)2A,                     in which            D is an alkylene radical,                        R is composed of ethylene radicals and propylene or butylene radicals in a ratio of ethylene radicals to the other alkylene radicals such that the ratio of carbon atoms to oxygen atoms in all blocks OR is in the range from 2.3:1 to 2.8:1,        z has an average value of from 25 to 100, and        A is a capping group,            (2) from 65 to 98% by weight of polypropylene glycol having an average molecular weight in the range from 1,000 to 2,000, and    (3) from 1 to 15% by weight of a hydrophobic silica.
Typical preparation methods for these aforementioned polyoxyalkylene-polysiloxane copolymers are described in U.S. Pat. No. 3,402,192, U.S. Pat. No. 3,746,653, U.S. Pat. No. 3,784,479, and U.S. Pat. No. 3,865,544.
According to the present state of knowledge the defoaming activity depends critically on the ability of a defoamer to penetrate the foam lamellae and thereby to destabilize them until they burst (K. Koczo, J. K Koczone, D. T. Wasan, J. Colloid Interface Sci. 166, 225 to 238 (1994)). For this to be achieved there must be a controlled incompatibility (hydrophobicity) with the aqueous phase in which defoaming is to take place. This is because a defoamer, if it is too compatible (hydrophilic), will no longer be able to be very effective, since it will not preferentially penetrate the foam lamella. If the incompatibility is too great, the defoaming is generally very good but is frequently accompanied in that case by unwanted side effects including surface defects, adverse effects on wetting behavior, and separation phenomena.
Accordingly, the search for a suitable defoamer always involves a search for the right balance between compatibility and incompatibility for the system in which defoaming is to take place, with the objective of coming as close as possible to the target hydrophobicity/hydrophilicity equilibrium. The ongoing concern to reduce VOC (volatile organic component) levels, coupled with the desire to prevent the fogging problems on interior walls that are caused by low-volatility mineral oils or conventional plasticizes by more effective formulating, have led to a situation where a large number of traditional product designs for the defoaming of aqueous coating systems nowadays appear unsuitable.
DE-A-40 32 006 teaches a method of defoaming and/or degassing organic systems by adding an antifoam comprising an organopolysiloxane to the organic system, which may consist of diesel oil or crude oil or products from the cracking of the oil. The organopolysiloxane used is a polymer composed of siloxane units of the general formulae
ti RaSiO(4−a)2 and RbAcSiO4−(b+c)2 
in which
    R is a monovalent hydrocarbon radical having 1 to 18 carbon atoms per radical,    A is a radical of the general formula      in which            R1 is a radical of the formula—CR3H—,                     R3 is hydrogen or a monovalent organic radical,            R is a radical of the formula—CR4H—CH3 or —(CH2)3,                             R4 is hydrogen or a monovalent organic radical,                                                v, w are each 0 or an integer, v+w being on average from 0 to 16,        x, y are 0 or 1, x+y being 1 or 2,            a is 1, 2 or 3,    b is 0, 1 or 2, and    c is 1 or 2, the sum b+c being not greater than 3.
The siloxanyl-alkenediyl-bis-ωhydroxypolyoxyalkylenes used themselves and their preparation are described in patent DD-A-2 55 737.
Adducts of alkynediol derivatives with hydrogen-functional siloxanes are therefore known.
From DE-A-195 16 30 and DE-A-43 43 235, moreover, derivatives are known which in addition to the alkynediol (alkoxylates) also describe other radicals, examples being polyether radicals, for the derivatization of the hydrogen siloxanes. These copolymers are also employed for defoaming apolar phases, such as diesel fuels.